Today the work spindles of many machine tools (above all, milling and grinding spindles, but also driven tool shafts on lathes, etc.) are in many cases implemented as motor spindles. With these, the actual work spindle, with bearing arrangement and electric drive as well as the tool holder with chucking system and release unit, internal lubricant feed, cooling etc., is combined into a compact drive assembly that is attached with screws or clamped to a feed carriage of said machine tool. As a result of a drive free of shear force or torsional play, the motor spindles feature, in comparison to conventional solutions having an external drive by means of couplings, belts or gears, the advantage of a greater running smoothness and fewer run-out errors, and have in addition, due to the lower rotating mass, shorter run-up times and braking times, with an improved workpiece surface quality being achievable together with simultaneously lower primary processing times. As a result of the high feed rates and accelerations, the mass of the spindle and of the feed carriage produces a kinetic energy that is substantially greater than the force from the feed drive, and can also not be diverted by means of a safety clutch at the feed shaft. For this reason, special protection measures must be met in order to protect the motor spindle from impact damage or the like. High-precision heavy-duty spindle ball bearings, in particular, can be damaged very quickly in the event of a collision caused by operator error or programming error; this is associated with considerable repair costs as well as lengthy machine downtimes for dismantling the motor spindle and installing a replacement spindle to be kept on hand.
Known from DE 195 27 561 A1 is an impact protection for a motor milling spindle of a machine tool, for which a work spindle with the entire bearing arrangement is arranged axially movable within a housing. The driveshaft of the motor is axially spaced apart from the work spindle, and is attached to it by means of a toothed coupling in order to transmit the torque of the motor. The work spindle is fixed within the housing axially by means of compression bushings. In the event of a collision, the work spindle together with the complete bearing arrangement can move along their longitudinal axis within the housing, with the compression bushings deforming plastically. This does indeed protect the bearing arrangement of the spindle, but after a collision the work spindle must be removed and disassembled in order to insert new compression bushings and to align the bearing arrangement again. In addition, the work spindle can move only in the direction of its longitudinal axis, and therefore is insufficiently protected from a lateral or oblique collision.